computeSourceRevision(self,
changes)

Each subclass must implement this method to do something more precise
than -rHEAD every time. For version control systems that use
repository-wide change numbers (SVN, P4), this can simply take the
maximum such number from all the changes involved in this build. For
systems that do not (CVS), it needs to create a timestamp based upon the
latest Change, the Build's treeStableTimer, and an optional
self.checkoutDelay value.